To benefit from special tax rates, there would need to be a system that ensures that once a special item was offered there would be no way to change the offered items.
Right now, you purchase access to a project. You do not purchase an eBook.
That on that project page currently is a single eBook does not change this. What if the publisher decides to put up a rpg game based on the rpg book on the same page? Or other digital files that do not have the same special tax rate. Or things like individually priced bonus files, a soundtrack maybe, on a pay what you want book project.
I assume those "legal documents" would include such a commitment for the project page.